The current methods used in forensic sciences for detecting vaginal epithelial cells in forensic samples are time consuming and presumptive. These methods are based on histological staining of glycogen-rich cells as vaginal cells. Typically a sample collected from a crime scene is used for DNA extraction and a separate portion of the sample is used for serology. Histological staining requires that cells collected from the crime scene are fixed to a microscopic slide, stained, dried, and analyzed by a microscopy expert. Moreover, the cells undergoing histological analysis are no longer viable to be used in other methods, for example, DNA extraction and analysis.
Further, the currently used methods based on histological staining of cells are prone to false results. For example, false negatives occur because the glycogen content of vaginal cells varies depending on the menstrual cycle and reproductive age. False positives can occur due the fact that buccal and urogenital skin cells (even from males) can have high content in glycogen and get stained.
Other methods currently used rely on the quantification of certain RNAs as a product of cell-specific gene expression. Gene expression at the transcription level is quantifiable by the levels of messenger RNA (mRNA) present in a specific type of cells or at the post-transcription level that occurs through micro RNA (miRNA) presence in specific tissues. Certain methods practiced to quantify mRNA or miRNA are capillary electrophoresis and high-resolution melt analysis. The use of RNA requires normalization of transcript levels with those of a housekeeping gene in the same sample. Often the normalization has to be performed by quantifying RNA levels prior to cDNA synthesis. Also, these methods do not discriminate between human and bacterial RNA, which is commonly present in most samples. Since most transcripts are present at a basal level in body fluids, the quantification of RNA for body fluid identification relies on levels of specific transcripts being higher than a certain threshold in specific cells. Lack of an accurate quantification of RNA in the sample leads to false results since it influences the threshold for the target transcript.